This invention relates to the production of ethylenically unsaturated acids or esters thereof, particularly methacrylic acid or alkyl methacrylates, and in particular to novel catalysts therefor. Such acids or esters may be made by reacting an alkanoic acid (or ester) of the formula Rxe2x80x2xe2x80x94CH2xe2x80x94COOR, where R and Rxe2x80x2 are each, independently, hydrogen or an alkyl group, especially a lower alkyl group containing for example 1-4 carbon atoms, with formaldehyde. Thus methacrylic acid or alkyl esters thereof, especially methyl methacrylate, may be made by the catalytic reaction of propionic acid, or the corresponding alkyl ester, e.g. methyl propionate, with formaldehyde in accordance with the reaction sequence:
CH3xe2x80x94CH2xe2x80x94COOR+HCHOxe2x86x92CH3xe2x80x94CH(CH2OH)xe2x80x94COOR
CH3xe2x80x94CH(CH2OH)xe2x80x94COORxe2x86x92CH3xe2x80x94C(:CH2)xe2x80x94COOR+H2O
The reaction is typically effected at an elevated temperature, usually in the range 250-400xc2x0 C., using a basic catalyst. Where the desired product is an ester, the reaction is preferably effected in the presence of the relevant alcohol in order to minimise the formation of the corresponding acid through hydrolysis of the ester. Also for convenience it is often desirable to introduce the formaldehyde in the form of formalin. Hence for the production of methyl methacrylate, the reaction mixture fed to the catalyst will generally consist of methyl propionate, methanol, formaldehyde and water.
Suitable catalysts that have been used include alkali metal-doped, especially cesium-doped, silica catalysts. It has been found that certain cesium-doped silica catalysts, i.e. those based upon gel silicas, have an unacceptable service life as they lose their activity and selectivity in a relatively short time. This activity loss may be attributed to two factors.
Firstly the alkali metal compound employed may exhibit appreciable volatility under the reaction conditions employed and so there may be a loss of activity through loss of alkali metal. As described in U.S. Pat. No. 4,990,662, this may be overcome by incorporating a suitable alkali metal compound into the process gas stream so that alkali metal compound is deposited on the catalyst during operation to compensate for any alkali metal compound lost through volatilisation.
Secondly, as may be inferred from U.S. Pat. No. 4,942,258, it is believed that for the alkali metal to be active, the support should have a certain minimum surface area. The requisite area is dependent on the amount of alkali metal in the catalyst: thus it may be inferred that there is a minimum surface area required per unit of alkali metal. During operation, there is a tendency for the silica support to lose surface area. Thus under the reaction conditions there is a risk of hydrolysis of the silica, not only by the water produced by the reaction, but also from water present in the reaction mixture, for example resulting from introduction of the formaldehyde as formalin. We have found that the loss of performance of the gel silica catalysts with time largely results from such hydrolysis causing a decrease in the surface area of the catalyst with time.
Typically the catalyst contains 1-10% by weight of the alkali metal. Preferably at least 2% by weight of alkali metal is employed so that the process can be operated at sufficiently low temperatures that loss of alkali metal through volatilisation can be minimised. The operation at low temperatures has the additional advantage that the rate of deposition of coke, which tends to block the pores of the silica and so reduce activity, is decreased.
We have found that the incorporation of certain modifiers, such as compounds of elements such as boron, aluminium, magnesium, zirconium, or hafnium into the catalysts, in addition to the alkali metal, retards the rate of surface area decrease. In the catalysts of the invention, it is important that the modifier is intimately dispersed in the silica, rather than simply being in the form of particles mixed with the silica particles. It is probable that the metal compounds in whatever form they are added will convert to oxides or (particularly at the surface of the silica) hydroxides before or during drying, calcination or operation of the catalyst and interact either on the surface or in the bulk of the silica structure in that form. Furthermore it is important that the amount of modifier is within certain limits: if there is too little modifier, no significant advantage accrues while if too much modifier is employed the selectivity of the catalyst may be adversely affected. Generally the amount of modifier required is in the range 0.25 to 2 gram atoms of the modifier element per 100 moles of silica.
The aforesaid U.S. Pat. No. 4,990,662 indicates that silicas may contain materials such as aluminium, zirconium, titanium, and iron compounds as trace impurities. That reference however indicates that improved catalysts are obtained if such impurities are removed by acid extraction to give a trace impurity content below 100 ppm.
EP 0 265 964 discloses the use of silica supported catalysts containing antimony as well as the alkali metal. The description indicates that the alumina content is desirably less than 500 ppm. A comparative, antimony-free, example discloses the use of a composition containing 950 ppm alumina. This corresponds to 0.11 gram atoms of aluminium per 100 moles of silica.
U.S. Pat. No. 3,933,888 discloses the production of methyl methacrylate by the above reaction using a catalyst formed by calcining a pyrogenic silica with a base such as a cesium compound, and indicates that the pyrogenic silica may be mixed with 1-10% by weight of pyrogenic zirconia. That reference also discloses the use of a catalyst made from a composition containing cesium as the alkali metal and a small amount of borax. The amount of boron however is about 0.04 gram atoms per 100 moles of silica and so is too small to have any significant stabilising effect. DE 2 349 054 C, which is nominally equivalent to U.S. Pat. No. 3,933,888, exemplifies catalysts containing zirconia or hafnia in admixture with the silica: the results quoted indicate that the zirconia or hafnia containing catalysts give a lower yield based upon the amount of formaldehyde employed.
Yoo discloses in xe2x80x9cApplied Catalysisxe2x80x9d, 102, (1993) pages 215-232 catalysts of cesium supported on silica doped with various modifiers. While bismuth appeared to be a satisfactory dopant, catalysts doped with lanthanum, lead or thallium gave short term improvements. However high levels of lanthanum gave products of low selectivity while low levels of lanthanum gave catalysts that sintered much faster than the bismuth doped catalysts. The effective additives were all highly toxic heavy metals with appreciable volatility: these considerations preclude their use as catalyst components.
The aforementioned U.S. Pat. No. 3,933,888 indicated that it was important to use a pyrogenic silica and showed that other types of silicas were unsuitable. The pyrogenic silicas said to be suitable are those having a total surface area in the range 150-300 m2/g, a total pore volume of 3-15 cm3/g and a specified pore size distribution wherein at least 50% of the pore content is in the form of pores of diameter above 10000 xc3x85 (1000 nm) and less than 30% is in the form of pores of diameter below 1000 xc3x85 (100 nm). In contrast, in the present invention the silicas that may be employed are preferably porous high surface area silicas such as gel silicas, precipitated gel silicas and agglomerated pyrogenic silicas.
Accordingly the present invention provides a catalyst comprising a porous high surface area silica containing 1-10% by weight of an alkali metal (expressed as metal), wherein the catalyst contains a compound of at least one modifier element selected from boron, magnesium, aluminium, zirconium and hafnium in such amount that the catalyst contains a total of 0.25 to 2 gram atoms of said modifier element per 100 moles of silica, said modifier element compound being dispersed in the pores of said silica.
The silica employed in the invention preferably has a surface area of at least 50 m2gxe2x88x921. The surface area may be measured by well known methods, a preferred method being a standard BET nitrogen absorption method as is well known in the art. Preferably the bulk of the surface area of the silica is present in pores of diameter in the range 5-150 nm. Preferably the bulk of the pore volume of the silica is provided by pores of diameter in the range 5-150 nm. By xe2x80x9cthe bulkxe2x80x9d of its pore volume or surface area is provided by pores of diameter in the range 5-150 nm we mean that at least 50% of the pore volume or surface area is provided by pores of this diameter and more preferably at least 70%.
Preferred alkali metals are potassium, rubidium, or especially cesium. The alkali metal content is preferably in the range 3-8%, by weight (expressed as metal).
Gel silicas are preferred although suitable pyrogenic silicas may also be used.
The preferred modifier elements are zirconium, aluminum of boron.
In an embodiment wherein boron is the modifier element the amount of boron may be from greater than 0.29 gram atoms to 2 grams per 100 moles of silica.
The invention also provides a process for the manufacture of ethylenically unsaturated acids or esters thereof, particularly methacrylic acid or alkyl methacrylates, by reaction of an alkanoic acid, or ester of an alkanoic acid, of the formula Rxe2x80x2xe2x80x94CH2xe2x80x94COOR, where R and Rxe2x80x2 are each, independently, hydrogen or an alkyl group, especially a lower alkyl group containing for example 1-4 carbon atoms, with formaldehyde in the presence of a catalyst as aforesaid.
The process is particularly suitable for the manufacture of methacrylic acid or especially methyl methacrylate, in which cases the alkanoic acid or ester is propionic acid or methyl propionate respectively.
Mixtures of modifier elements may be used, for example aluminium and zirconium, or magnesium and zirconium. The total amount of the modifier element in the catalyst is preferably in the range 0.25 to 1.5 gram atoms per 100 moles of silica. Too little modifier element generally results in inadequate stabilisation of the silica support, leading to loss of activity through loss of surface area, while too much modifier element often leads to a decrease in the selectivity of the catalyst.
The catalysts may be made by impregnating silica particles of the physical dimensions required of the catalyst with a solutions of a suitable compounds, e.g. salts, of the modifier element in a suitable solvent, followed by drying. The impregnation and drying procedure may be repeated more than once in order to achieve the desired additive loading. As there appears to be competition between the modifier and alkali metal for active sites on the silica, it may be desirable for the modifier to be incorporated before the alkali metal. We have found that multiple impregnations with aqueous solutions tend to reduce the strength of the catalyst particles if the particles are fully dried between impregnations and it is therefore preferable in these cases to allow some moisture to be retained in the catalyst between successive impregnations. When using non-aqueous solutions, it may be preferable to introduce the modifier first by one or more impregnations with a suitable non-aqueous solution, e.g. a solution of an alkoxide or acetate of the modifier metal in ethanol, followed by drying and then the alkali metal may be incorporated by a similar procedure using a solution of a suitable alkali metal compound. Where aqueous solutions are employed, it is preferable to effect the impregnation using an aqueous solution of e.g. nitrates or acetates of the modifier metal and cesium of sufficient concentration that the desired loading of modifier and cesium is effected in a single step, followed by drying.
The modifier elements may be introduced into the silica particles as soluble salts but we believe that the modifier element(s) are present in the silica in the form of oxides and /or hydroxides (especially at the surface of the silica) which are formed by ion exchange during impregnation, drying, calcining or catalytic use of the catalyst.
Alternatively the modifier may be incorporated into the composition by co-gelling or co-precipitating a compound of the modifier element with the silica, or by hydrolysis of a mixture of the modifier element halide with a silicon halide. Methods of preparing mixed oxides of silica and zirconia by sol gel processing are described by Bosman et al in J Catalysis Vol 148 (1994) page 660 and by Monros et al in J Materials Science Vol 28, (1993), page 5832. Doping of silica spheres with boron during gelation from tetraethyl orthosilicate (TEOS) is described by Jubb and Bowen in J Material Science, volume 22, (1987), pages 1963-1970. Methods of preparing porous silicas are described in Iler R K. The Chemistry of Silica, (Wiley, N.Y., 1979), and in Brinker C J and Scherer G W xe2x80x9cSol-Gel Sciencexe2x80x9d published by Academic Press (1990). Thus methods of preparing suitable silicas are known in the art.
The catalysts are then preferably calcined, for example in air, at a temperature in the range 300 to 600xc2x0 C., particularly at 400-500xc2x0 C. before use, although we have found that this may not always be necessary.
The catalysts will normally be used in the form of a fixed bed and so it is desirable that the composition is formed into shaped units, e.g. spheres, granules, pellets, aggregates, or extrudates, typically having maximum and minimum dimensions in the range 1 to 10 mm. Where an impregnation technique is employed, the silica may be so shaped prior to impregnation. Alternatively the composition may be so shaped at any suitable stage in the production of the catalyst. The catalysts are also effective in other forms, e.g. powders or small beads and may be used in this form.
The alkanoic acid or ester thereof and formaldehyde can be fed, independently or after prior mixing, to the reactor containing the catalyst at molar ratios of acid or ester to formaldehyde of from 20:1 to 1:20 and at a temperature of 250-400xc2x0 C. with a residence time of 1-100 seconds and at a pressure of 1-10 bara. Water may be present up to 60% by weight of the reaction mixture, although this is preferably minimised due to its negative effect both on catalyst decay and hydrolysis of esters to acids. Formaldehyde can be added from any suitable source. These include but are not limited to aqueous formaldehyde solutions, anhydrous formaldehyde derived from a formaldehyde drying procedure, trioxane, diether of methylene glycol and paraformaldehyde. Where forms of formaldehyde which are not as free or weakly complexed formaldehyde are used, the formaldehyde will form in situ in the synthesis reactor or in a separate reactor prior to the synthesis reactor. Thus for example, trioxane may be decomposed over an inert material or in an empty tube at temperatures over 350xc2x0 C. or over an acid catalyst at over 100xc2x0 C. As a second example, methylal may be decomposed by reaction with water to form formaldehyde and methanol or without water to form dimethyl ether and formaldehyde. This can be accomplished either within the reactor or in a separate reactor containing a catalyst such as an heterogeneous acid catalyst. In this case it is advantageous to feed the alkanoic acid or ester thereof ester separately to the synthesis reactor to prevent its decomposition over the acid catalyst.
When the desired product is an unsaturated ester made by reacting an ester of an alkanoic acid ester with formaldehyde, the alcohol corresponding to the ester may also be fed to the reactor either with or separately to the other components. The alcohol, amongst other effects, reduces the quantity of acids leaving the reactor. It is not necessary that the alcohol is added at the beginning of the reactor and it may for instance be added in the middle or near the back, in order to effect the conversion of acids such as propionic acid, methacrylic acid to their respective esters without depressing catalyst activity.
Other additives may be added either as inert diluents to reduce the intensity of the reaction or to control heat evolution from the catalyst as a result of reaction. Reaction modifiers may also be added, to for instance change the rate of carbon laydown on the catalyst. Thus for instance oxidising agents such as oxygen may be added at tow levels to reduce the rate of coke formation. Additives may also be included to aid separations by for instance changing the composition of an azeotrope. Whilst such components to achieve the latter effect may be advantageously added after the reactor, in some circumstances it may be advantageous to include the additive in the reactor.
In order to minimise the loss of alkali metal through volatilisation, alkali metal, in a suitable form, e.g. a volatile salt, may be continuously or intermittently fed to the reactor.
The invention is illustrated by the following examples.